Seems to me you are finding that the larger the aperture with near objects there
might be a limit to what's possible with quality of render. Such that a real
camera would allow a smooth transition if something were next to its lense, but
POV-Ray can't (still guessing here) retain the defocusing in a likewise smooth
manner. Hence your dividing bars problem seen before.

I suspected you might have been dealing with a large scene scale, obviously not
the case after seeing your test scene.
Mine used object to camera distances of 10 and 100 POV Units, and since then I
have tried weaving together those with the aperture to see the effect of scene
scale. Maybe not what you asked about, but it is a relationship of distances and
aperture size.

Posting that here for you so you know what I'm using to check this with. Oh, and
a hint for improving focal blur quality is to use +a (default antialiasing)
along with these type of renders. I realize the documentation says camera blur
doesn't need that but it can help clean it up anyhow.

Most importantly is that I believe you are probably going to find limitations
concerning scene dimensions and camera focus. However, think of it this way... a
real camera lens tends to lose depth of field when everything is a great
distance away. And when things are very close they all but disappear if way out
of the focal plane, just not entirely so there's a ghosting effect from those
close objects.

Anyway I'm fairly sure POV-Ray won't cover every possible combination of
camera+object distances with the same accuracy of actual camera lenses.

I know this subject was discussed again and again over the many years since
focal blur was introduced, unfortunately I failed to find what I was searching
for. I was thinking someone had at some time had attempted to relate POV-Ray to
lens focus long before you asked here recently. Sorry I couldn't.

Bob

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